add BalanceDelta reference guide with mdx formatting

[krisoshea-eth]

This PR adds a reference guide for the BalanceDelta type in Uniswap V4.

Key Features of the Document:

1. Introduction to BalanceDelta and its role in Uniswap V4
2. Detailed explanation of the purpose and importance of BalanceDelta in managing pool balances
3. Type definition and using directives for BalanceDelta
4. Comprehensive breakdown of core functions:
   • toBalanceDelta
   • add
   • sub
   • eq
   • neq
5. Explanation of library functions:
   • amount0
   • amount1
6. Discussion on the usage of BalanceDelta in hooks and the Pool library
7. Insights into how BalanceDelta contributes to gas optimization in Uniswap V4 operations

Document Structure:

• Uses MDX formatting with frontmatter
• Provides Solidity code snippets for type definitions and function signatures
• Includes tables for function parameters
• Follows a logical flow from concept introduction to implementation details and usage scenarios

Impact and Importance:

Understanding BalanceDelta is essential for implementing custom hooks, managing pool balances, and optimizing gas usage in Uniswap V4 operations. It provides insights into how Uniswap V4 efficiently handles balance changes across various pool operations.

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